Multiple-input, multiple-output (MIMO) techniques can significantly increase system capacity in a scattering environment of a wireless network. However, the use of more antennas increases the hardware complexity and cost because in a typical system, each transmit/receive antenna requires a separate RF chain including a modulator/demodulator, an AD/DA converter, an up/down converter, and a power amplifier. In addition, the processing complexity at the baseband also increases with the number of antennas.
Antenna/beam selection can reduce the number of RF chains while still taking advantage of the capacity/diversity increase provided by multiple antennas. In a wireless local area network (WLAN), stations are typically operated at a high signal to noise ratio (SNR), in which diversity plays a key role in protecting the system from a deep fading channel. Furthermore, it is known that the state of a WLAN channel changes slowly. Therefore, it is advantageous to perform antenna/beam selection in a WLAN.
The idea of antenna/beam selection is to select a submatrix from a complete channel matrix or a transformed channel matrix for beam selection, according to some predetermined criterion. To perform antenna/beam selection, the complete channel matrix is estimated by sending training (sounding) frames that enable the antenna selection station to measure the entire channel state. Conventionally, explicit signaling is used in the physical (PHY) or media access (MAC) layer by sending training frame(s) for all the antennas to be selected. However, the additional overheads are undesirable due to practical limitations. On the other hand, the slowly varying WLAN channel environment can advocate a more efficient antenna/beam selection training scheme which requires little or no changes in the MAC and PHY layers.
Structure of IEEE 802.11n WLAN Link Adaptation Control (LAC) Frame in MAC Layer
As shown in FIG. 1, the WLAN IEEE 802.11n standard, incorporated herein by reference, also known as WiFi, specifies a link adaptation control frame (LAC) defined at the MAC layer for supporting MIMO training requests and exchange of link adaptation information. In general, the control frame contains the following fields: a MAC header 110, a LAC mask 120 for indicating the logical elements carried in the current control frame, a modulation coding scheme (MCS) feedback field 130 for indicating transmitting parameters, and a frame check sequence (FCS) 140 for error detection. The MAC header 110 applies for any MAC layer packet, which includes a frame control 111, duration 112, receive address (RA) 113, and transmit address (TA) 114. The control frame is described in detail in IEEE 802.11-04/0889r7, “TGn Sync Proposal Technical Specification,” incorporated herein by reference.
The LAC frame supports control of MIMO training requests and exchange of link adaptation information. The LAC frame can be sent by either an initiator station (transmitter) or a recipient station (receiver).
FIG. 2 shows the LAC mask field 120 in greater detail. Without considering antenna/beam selection, the LAC mask field 120 includes the following: RTS (request to send) 121, CTS (clear to send) 122, TRQ (MIMO training request) 123, MRQ (request for MCS feedback) 124, and MFB (MCS feedback) 125. The three bits 126 are reserved. In the MCS feedback case, i.e., MFB=1, the MCS set is indicated in the ‘MCS feedback’ field 130 in FIG. 1.
Closed-Loop MIMO Training Methods for IEEE 802.11n WLAN
The IEEE 802.11n standard requires a throughput of 100 megabits per second (Mbps) at the medium access control (MAC) layer service access point (SAP). Based on the channel property in WLAN environment, closed-loop schemes are preferred for increased throughput, including transmit beam forming (TXBF), MCS adaptation, and antenna/beam selection.
Each PHY layer packet is composed by two portions: preamble and data. The PHY packet preamble includes training information for channel estimation at the receiver. Typically, in a conventional PHY layer packet, the number of antennas or spatial streams indicated in the training field can be less than the maximum number provided by the MIMO channel. A sounding packet is a specific PHY layer packet, which contains the training information for all the available data streams in the MIMO channel, no matter how many data streams are used for transmitting the data portion. When the concept of sounding packet is not applied in the system, an alternative category of PHY layer training packet is the one that enforces a MCS set utilizing all the available data streams in the MIMO channel, so that not only the preamble contains the full training information of the MIMO channel, the data portion is also transmitted using all the available data streams.
MCS Training Process
FIG. 3 shows a conventional MIMO training process for MCS adaptation. An initiator (transmit) station STA A 301 sends a LAC frame 310 with MRQ=1, to a recipient (receive) station STA B 302. The initiator also requests its PHY layer to signal a sounding packet. In response to receiving the MRQ and the sounding packet, the recipient 302 estimates the MIMO channel and determines an appropriate MCS set for the current channel. Then, the recipient replies to the initiator a LAC frame 320 with MFB set to 1, and the MCS feedback field 130 contains the selected MCS set.
The recipient 302 can also initiate the MCS training process whenever it has the complete MIMO channel knowledge, by determining the MCS and sending an MFB with MCS feedback directly without any matching MRQ element. This is called unsolicited adaptation.
TXBF Training Process
FIG. 4 shows a conventional transmit beam forming (TXBF) training process. The initiator 301 sends out a LAC frame 410 with TRQ set to 1 to the recipient 302. In response to receiving the TRQ, the recipient sends back a sounding packet 420 to the initiator. Upon receiving the sounding packet, the initiator estimates the MIMO channel and updates its beam forming steering matrices. Up to now, recipient initiated TXBF training is not defined.
For antenna selection, some prior art training methods use a single PHY layer training frame (e.g., sounding packet) containing the training information for all the antennas to be selected, and different antenna subsets are subsequently connected to the RF chains for this single training frame. This introduces overhead on existing training frame designs.
In another training method, a long sequence of training frames is transmitted from a receive station to a transmit station, and in response the transmit station transmits a short sequence of training frames so that both the transmit and receive station can perform channel estimation and antenna selection, see U.S. patent application Ser. No. 11/127,006 “Training Frames for MIMO Stations,” filed by Andreas Molisch, Jianxuan Du and Daqing Gu on May 11, 2005, incorporated herein by reference.